1. Field of the Invention
The present invention relates to a fluorescent display device provided with one or more anodes having fluorescent layers deposited on the upper surfaces thereof for luminous display in the form of numerical letters, figures or the like upon impingement of thermions emitted from cathode.
2. Description of the Prior Arts
Conventional fluorescent display devices are provided with a plurality of anodes having fluorescent layers deposited on the upper surfaces thereof and arranged in any suitable pattern depending upon the purpose of display, and adapted to perform luminous display in the form of letters, figures or the like, by impinging thermions emitted from filamentary cathode selectively on the above anodes thereby exciting the fluorescent layers to emit light.
The fluorescent layers deposited on the anodes of the above fluorescent display devices are generally composed of a ZnO:Zn system fluorescent material. Such fluorescent material has several advantages in use such as a very low luminous threshold voltage, that is, a dead voltage as low as about several volts, drivability at a low voltage and easy observation for its green luminous color.
Consequently, the foregoing fluorescent display devices are advantageous in that they can be driven at a low voltage with less power consumption and provide luminous displays which are easy to observe. Thus, the fluorescent display tube has been extensively used in digital display systems such as numerical displays in various types of electronic devices, for example, electronic table computers and digital clocks, frequency indication in wireless units or the like, and in analog display systems such as level indicators in acoustic units, speedometers for vehicles and air crafts, as well as display sections in various instruments. The recent extensive use of the fluorescent display devices requires devices which are capable of emitting luminous colors other than green, or those devices whose luminous colors can be varied portionwise in one display device depending on specified uses. In order to meet with the above requirements, various fluorescent materials for low speed electron ray use have been developed and have come into practical use which can be excited to emit luminous lights under a driving voltage approximately at the same level as that for the ZnO:Zn system fluorescent material. In addition, a fluorescent material of red luminous color consisting of Y.sub.2 O.sub.2 S:Eu or Y.sub.2 O.sub.3 :Eu incorporated with an appropriate amount of In.sub.2 O.sub.3 or the like to reduce the resistivity for the adaption to the low speed electron ray use, and a fluorescent material of yellow luminous color for low speed electron ray use consisting of ZnS:Mn, Cl incorporated with ZnO or the like have been developed.
The foregoing display devices whose luminous color can be varied portionwise in one device have thus been put to practical use with the advent of those fluorescent materials emitting luminous light of colors other than green under the irradiation of low speed electron rays. A test production has been attempted for those fluorescent display devices in which the above improved fluorescent materials are employed in an analog display device such as a speedometers, so that the display for an excess input or the display for a speed in excess of regulated level is indicated as a color different from that for other display areas to thereby improve the warning effect in the display.
In the above display devices where luminous display regions of different colors are formed portionwise in one display device, however, the respective display regions providing luminous colors are predetermined. Consequently, while the above system having display regions with luminous colors formed portionwise is well suited to analog devices which supply warning effects and information regarding the excess from a regulated level, for example, the level indicator or the speedometer, such system is often inapplicable to fluorescent display devices in which a digital display is produced by means of segmented anodes deposited with the fluorescent layers and arranged in the form of the letter "8". Thus, it is required in the display devices for displaying such numerical letters, figures or the like to vary the luminous color over the entire display pattern not restricted to the specified regions. Highly effective warning operation can be expected also in the analog display device if the luminous colors can be changed for the entire display pattern.
While on the other hand, conventional warning devices generally employed thus far in display devices for analog indication include those using a warning color, generally red or the like, in specified display areas or instruction sections, for example, for numeric values combined with such display areas corresponding to analog display information that require warning, or those adapted to light up warning lamps disposed in combination with or separately from the above warning devices or buzzers.
The conventional warning devices of the foregoing types have, however, several disadvantages. Those adapted to conduct warning by the use of the warning color previously set to the warning means are often inapplicable to the display device employing digital type fluorescent tubes whose application use has rapidly been increased recently, since the warning colors are always displayed. The warning devices utilizing warning lamps or buzzers require additional provision of such lamps or buzzers independent of the display devices and, if the lamps or the buzzers should fail, perform no warning function at all. Moreover, since the warning devices are not always arranged in correspondence with the display sections, retardation may result in response to the warning or unnecessary psychological burdens may be imposed on the driver.